Information Technology Reference
In-Depth Information
Fig. 3.30 Scheme of a
planar molecular memory
on the basis of the
bacteriorhodopsin protein
molecule behaves as a system with two stable states, bR(570) and K(610), with
transitions between them initiated by light in the visible spectral range.
In the early 1990s, based on this property of bacteriorhodopsin, Robert Birge
developed a cryogenic optical random access memory for digital computers. In this
device information was recorded on a film made of bacteriorhodopsin-containing
polymer and read from it by laser beams with various frequencies of radiation
(Fig.
3.30
). The memory with 25 MB of storage capacity and access time of 10-
100 ns was operated at 77 K. At the same time, 25 MB cache storage was
developed, operating at temperatures close to room temperature, in which a differ-
ent excited state of bacteriorhodopsin was used.
During the 1990s and in the beginning of this century, the Birge group has also
developed a holographic associative memory capable of reading out information
with only a part of it being available (e.g., reconstructing an image based on a
fragment). But the main efforts of the group were directed at the development of
ultrahigh-capacity memory.
Unlike earlier versions of storage devices in which information was recorded on
a flat bacteriorhodopsin film, in the volumetric memory, as its name implies, the
entire volume of the medium is used to store information.
It should be said that the bacteriorhodopsin-based volumetric memory is a
technically sophisticated electron-optical device. It includes several lasers emitting
in different spectral regions (Fig.
3.31
).
In a memory device, the main bR-state and the Q-state of the bacteriorhodopsin
molecule correspond to the digit values of “0” and “1,” respectively. These long-
lived states can remain unchanged for several years. The storage medium is a
cuvette 1
1
2 in. in size filled with polyacrylamide gel with embedded bacte-
riorhodopsin molecules. Around the cuvette lasers and devices that convert and
record light information are situated.
Consider a simplified process for recording and reading information in the
volumetric memory (Fig.
3.32
). The process of writing begins with the selection
and activation of a thin layer (page) within the volume of the storage medium. The
thickness of the page can vary from 15 to 100
m. From the information point of
view, the page is a quasi-flat storage media with 4,096
4,096 bits capacity. The
system is activated by green lasers. They excite all bacteriorhodopsin molecules
located in the medium layer constituting the page. After about 2 ms, the concen-
tration of molecules existing in the O-state on the page reaches a maximum. At this
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